Such recorders comprise one or more magnetic read/write heads fixed at the periphery of a rotary element lying coaxially with the axis of a cylindrical drum, with a magnetic tape being wound helically around the surface of the drum and running from a payout spool to a takeup spool.
Recorders of this type constitute two families. In the first family, the heads are fixed on the periphery of a roating plate which is coaxial with the drum and they are disposed in a slot provided in an equatorial plane of the drum. Said magnetic tape wound helically round the drum runs past the slot. In the other family, the drum comprises a fixed bottom portion and a rotating top portion to which the heads are fixed.
The inclination of the tracks on the tape is mainly a function of the inclination of the tape relative to the plane of rotation of the heads.
In general, the inclination of the tracks is fixed by an official or a de facto standard.
Consequently, information recorded on the tape must be disposed thereon accurately, since otherwise, when reading, the heads no longer follow the tracks as recorded on the tape.
This assumes that the tape is guided accurately so that the tape runs over exactly the same location of the drum both when recording and when reading (particularly when recording and reading are performed on different apparatuses), or even during a single recording or reading operation (using the same apparatus).
By way of indication, a track is a few tens of microns wide.
It will be understood how important it is to guide the tape over the drum, with guidance being provided with an accuracy of a few microns.
Further, the continuing improvement in the performance of such recorders leads, in particular, to an increase in the quantity of information that can be processed. Thus, for given dimensions of the reel on which the tape is wound, it is desired to reduce the thickness of the tape or at least the layer of material constituting its backing, in order to increase the total length of wound tape.
It is normal practice for such thin backings to be made by stretching a thicker layer of backing material. This gives rise to thicknesses of a few tens of microns, or even of ten microns.
However, tapes of this type give rise to a major difficulty.
A tape which has been subjected to such treatment expands thermally in an anisotropic manner. Its coefficient of expansion is generally five times higher in the transverse direction than in the longitudinal direction.
As a result, the angle of inclination of the tracks on the tape varies with temperature. Thermal expansion gives rise to tape displacement relative to the drum both in translation and in rotation.
Consequently, if there is a change in temperature (e.g. between recording and reading, or even while recording), then the tracks can no longer be read back properly by the heads.
This give rise to a loss of information and thus a considerable reduction in recording reliability.
This drawback is totally unacceptable, in particular when temperatures vary over a wide range.
In prior art devices, the tape is guided either by means of wheels/pins disposed at the inlet and at the outlet of the drum, or else by a helical ramp applied to the drum or machined in the body thereof with one of the top or bottom edges of the tape bearing against the edge of the ramp.
However, none of the prior art devices solve the above-mentioned problem due to anisotropic thermal expansion of the tape.
The device of the invention seeks to remedy this drawback.